Conventional low density polyethylene (LDPE) has good processability; however, when used in film application, increased melt strength is still desired. International Publication WO 2012/084787 discloses a process for preparing ethylene copolymers in the presence of free-radical polymerization initiator, at pressures in the range of from 160 MPa to 350 MPa, and temperatures in the range of from 100° C. to 350° C., in a tubular reactor by copolymerizing ethylene, a bi- or multifunctional comonomer and optionally further comonomers. The bi- or multifunctional comonomer bears at least two different functional groups, of which at least one is an unsaturated group, which can be incorporated into the growing polymer chain, and at least another functional group can act as chain transfer agent in radical ethylene polymerization.
International Publication No. WO 2006/094723 discloses a process for the preparation of a copolymer of ethylene and a monomer copolymerizable therewith. The polymerization takes place in a tubular reactor at a peak temperature between 290° C. and 350° C., the comonomer is a di- or higher functional (meth) acrylate and the comonomer is applied in an amount between 0.008 mol % and 0.200 mol % relative to the amount of ethylene copolymer.
JP61028685B2 (Abstract) discloses straight chain ethylene random copolymers containing ethylene, and a polyalkylene glycol monoacrylate and other ethylenic unsaturated monomers. These ethylene random copolymers are disclosed as having excellent hygroscopicity and antistatic properties, and compatibility with polyolefins.
International Publication WO 2014/003837 discloses an ethylene-based polymer formed from reacting at least the following: ethylene and at least one asymmetrical polyene, comprising an “alpha, beta unsaturated end” and a “C—C double bond end,” and wherein the reaction takes place in the presence of at least one free-radical initiator.
U.S. Pat. No. 4,370,517 discloses preparing a graft polymer of polyethylene and a polyfunctional monomer such as methoxy-polyethylene glycol methacrylate or polyethylene glycol dimethacrylate for use in electrical cable insulation for the suppression of water tree formation. U.S. Publication No. 2008/0242809 discloses a process for the preparation of a copolymer of ethylene and a comonomer, and where the polymerization takes place in a tubular reactor, at a peak temperature between 290° C. and 350° C. The comonomer is a di- or higher functional (meth)acrylate, and the co monomer is used in an amount between 0.008 mole percent and 0.200 mole percent, relative to the amount of ethylene copolymer.
U.S. Pat. No. 5,539,075 discloses the polymerization of ethylene and at least one monomer, which is copolymerizable with ethylene, and includes a polyunsaturated comonomer having a chain of at least eight carbon atoms and at least two non-conjugated double bonds, of which at least one is terminal. The polymerization takes place at a pressure of about 100-300 MPa, and a temperature of about 80°-300° C., under the action of a radical initiator. The polyunsaturated comonomer preferably is an α,ω-alkadiene having 8-16 carbon atoms, and most preferred 1,9-decadiene. Apart from the polyunsaturated comonomer, the polymerization may also involve another vinyl-unsaturated monomer, preferably containing at least one functional group selected from hydroxyl groups, alkoxy groups, carbonyl groups, carboxyl groups and ester groups.
Additional polymers and processes using comonomers and/or agents that affect polymer rheology are disclosed in the following references. International Publication Nos. WO 2007/110127, WO 1997/045465, WO 2012/084787, WO 2015/100302, WO 2015/100318, WO 2015/100351, WO 2015/200426, WO 2015/200430; U.S. Pat. No. 3,542,749; U.S. Patent Publication 2013/0237678; U.S. Patent Publication 2009/0253878; EP 1999170A1; Tung, L. H., et al., Preparation of Polystyrene with Long Chain Branches via Free Radical Polymerization. J. Polym. Sci., Polym. Chem. Ed., (1981), 19, 2027-39; Tung, L. H., Branching Kinetics in Copolymerization of Styrene with a Chain -Transfer Monomer , J. Polym. Sci., Polym. Chem. Ed., (1981), 19, 3209-3217; Liu, J., et al., Branched Polymer via Free Radical Polymerization of Chain Transfer Monomer: A Theoretical and Experimental Investigation , J. Polym. Sci. Part A: Polym. Chem., (2007), 46, 1449-59.
However, as discussed, there remains a need for ethylene-based polymers, such as high pressure (≥100 MPa), free-radical polymerized ethylene-based polymers, with improved melt strength, especially for film and extrusion coating applications. This need and others have been met by the following invention.